1. Field of the Invention
The present invention relates to a device for successively transferring or conveying electronic components one after another, and a system which is equipped with such an electronic component transferring device and adapted to mount the electronic components one at a time on a workpiece such as a printed-circuit board.
2. Discussion of the Related Art
A device for successively transferring electronic components one after another is required, for example, in an electronic component mounting system adapted to mount the electronic components on a printed-circuit board (PC board) or other workpieces. As one type of such an electronic component transferring device, there is known a device using a rotary table which carries a plurality of component holder heads such that the heads are evenly spaced from each other at a predetermined angular interval, which corresponds to an angular pitch of intermittent rotation of the rotary table. As the rotary table is intermittently rotated, the electronic component held by each component holder head is transferred. When the rotary table is stopped, one of the heads which is located at a component receiving position receives an electronic component from a suitable component supply device, while another head placed in a component transfer position transfers an electronic component to a suitable component receiving device.
However, the known electronic component transferring device of the type described above suffers from a problem that the component holder heads are unnecessarily stopped at some angular positions, leading to a low operating efficiency of the device. Described in detail, all of the component holder heads are moved and stopped together during intermittent rotation of the rotary table. Actually, each component holder head is required to be stopped at only two angular positions, namely, at the component receiving position and the component transfer position, but the head has to be stopped at each of the predetermined angular positions which are spaced from each other at the angular spacing pitch of the component holder heads. It is also noted that a drive device for rotating the rotary table should accelerate and decelerate a relatively large initial mass consisting of the rotary table and the plurality of component holder heads carried by the rotary table. Accordingly, the drive device should have a comparatively large capacity, and tends to be expensive. Further, the device is likely to generate undesirable noises and vibrations due to acceleration and deceleration of a relatively large inertial mass as described above.
The number of the component holder heads carried by the rotary table may be made equal to the number of the stop positions at which each head is to be stopped. This arrangement is free from unnecessary stopping of the component holder heads. This arrangement, however, results in an increase in the rotary movement pitch of the component holder heads, leading to another problem that the time required for one rotary movement of each head is undesirably increased. In this respect, it is noted that the diameter of a circle described by each component holder head during rotation of the rotary table should be larger than a given lower limit, to permit the electronic component transferring device to have required size and space and achieve the intended purpose of transferring the electronic components. Consequently, a decrease in the number of the stop positions of the component holder heads inevitably results in an increase in the pitch of the rotary movement of the rotary table.
In the light of the above drawback, there are proposed improved electronic component transferring devices as disclosed in JP-A-6-77693 (published Mar. 18, 1994) and JP-A-6-45787 (published Feb. 18, 1994). In these improved devices, a plurality of component holder heads are carried by a rotary table such that the heads are evenly spaced from each other at a predetermined angular interval, as in the known device described above. Unlike the conventional device, however, the improved devices are adapted such that the rotary table is continuously rotated at a given speed, while the heads that are required to be stopped at the desired positions such as the component receiving and transfer positions are rotated about the axis of the rotary table in the direction opposite to the direction of rotation of the rotary table, at the same speed as the rotating speed of the rotary table, so that these heads are virtually stopped at the desired stop positions.
In the arrangement described above, only the component holder heads that should be stopped are stopped, while the other heads are kept rotated at a constant speed with the rotary table. Accordingly, the required capacity of the drive device for the rotary table can be reduced, leading to reduced size and cost of the drive device and reduced operating noises and vibrations of the device. In addition, the time required for each component holder head to reach the component receiving and transfer positions can be reduced. In the conventional device, the time required for each intermittent rotary movement of each head corresponding to the angular spacing pitch of the heads consists of a time required for accelerating the head from zero, a time required for decelerating the head to zero, and a time during which the head is held stopped. Therefore, the minimum required total time for each intermittent rotary movement of the head about the axis of the rotary table to the component receiving and transfer positions is inevitably long. In the electronic component transferring devices disclosed in the above-identified publications, on the other hand, each head is moved at the constant speed into an angular pitch region in which the stop position is located, and thereafter the head is decelerated to zero and is held stopped at the stop position, unless the head is required to be stopped at two successive stop positions which are adjacent angular pitch positions of the rotary table. This arrangement provides a time saving for each intermittent rotary movement, which is substantially equal to a sum of a difference between the time required for the rotary movement at a constant speed over a given angular range and the time required for the movement over the same angular range with acceleration to the above-indicated constant speed, and a half of the predetermined stop time. Accordingly, the time required for the head to reach the desired stop position is reduced.
While the rotary table used in the above improved devices has a relatively large mass, the rotary table is rotated at a constant speed, and the component holder heads which are provided on the rotary table and which are accelerated and decelerated for stopping at the desired stop positions have a relatively small mass. Therefore, the operating noises and vibrations are relatively small even when the heads are moved at relatively high acceleration and deceleration values, and the operating efficiency of the device is improved. Further, since the mass to be accelerated and decelerated is comparatively small, the inertial force at given acceleration and deceleration values is comparatively small, and the required capacity of the drive device for rotating the heads about the axis of the rotary table is accordingly small.
However, the electronic component transferring devices disclosed in the above-indicated publications suffer from some problems due to stopping of the component holder heads at the stop positions by rotating the heads about the axis of the rotary table in the direction opposite to the direction of rotation of the rotary table while the rotary table is rotated continuously. That is, the conventional arrangement disclosed in the publications suffers from difficulty to completely stop the heads at the stop positions, and difficulty to improve the accuracy of positioning of the heads at the stop positions. That is, the heads cannot be completely stopped unless the rotating speed of the heads is exactly the same as that of the rotary table. To assure high accuracy of positioning of the heads at the stop positions, it is required to accurately control the point of time at which the speed of the rotation of the head relative to the rotary table in the reverse direction has been raised to a constant level after initiation of this rotary movement of the head. However, it is difficult to accurately control this timing.
Further, suitable devices should be provided between the rotary table and the individual component holder heads, for rotating the heads about the axis of the rotary table. Since these devices are required to accurately stop the heads at the desired stop positions, the cost of manufacture of these devices is inevitably increased.